1. Field of the Invention
The invention concerns a method for calibration of a thickness gauge, in which the thickness gauge measures the thickness of a measured object in a stipulated measurement direction with at least one displacement sensor, operating contactless or by scanning, a reference object with known thickness and shape being brought into at least one partial area of the measurement field of the at least one displacement sensor.
2. Description of Related Art
A method for calibration of a thickness gauge with two displacement sensors operating contactless or by scanning is known from DE 40 11 717 A1, in which a reference object with known thickness is brought into the measurement field of the displacement sensors. In the known method, two sensors measure the distance to the top and bottom of the reference object from the top and bottom of the reference object in a stipulated measurement direction. These two independent measured values in the form of determined distance values are subtracted from the known distance of the sensors from each other, so that the thickness of the reference object is obtained. Since the sensors ordinarily exhibit non-linearities in their measurement behavior, the reference object is moved back and forth in the axial direction of the sensors, i.e., in the measurement direction, between the sensors, so that several thickness measurements can be made as a function of the position of the reference object. In other words, the distance between the displacement sensors and the reference object or the thickness of the reference object can be measured in any number of relative positions of the reference object and the deviation of sensor measured values from the stipulated known thickness of the reference object resulting from the non-linearity of the displacement sensors for each relative position can be recorded as the measurement error assigned to the corresponding sensor measured value and stored. During a subsequent thickness measurement of any object, the non-linearities of the displacement sensors can therefore be compensated over the entire measurement range.
In the known methods, it is problematical that during calibration it is of great significance that the reference object must always be moved back and forth very exactly without tilting in the axial direction of the opposite sensors. Tilting of the reference object causes a significant distortion of the measured values as a function of the tilting angle. A much greater presumed thickness is measured in a tilted reference object than in an untilted reference object. Tilting of only 5° already results in a measurement error of 0.38%, which is already much too high for many precision measurements, where layer thicknesses of sheets, films or the like are monitored.
To guarantee exact calibration and measurement, both the reference object and the actual measured object must therefore be guided precisely parallel, which is only very difficult in practice or often even impossible.
Another drawback of the known method consists of the fact that slightly different thicknesses of the reference object, which occur, for example, because of surface roughness or local unevenness, distort the calibration.
The underlying task of the present invention is therefore to provide a method for calibration of a thickness gauge, with which a particularly accurate and simple calibration is made possible.